The performance and mission applications of future ramjet and scramjet powered vehicles are highly dependent on protecting the engines and airframe from high heat loads encountered at hypersonic speeds. As aircraft flight speeds increase to the high supersonic and hypersonic regimes, aerodynamic heating becomes increasingly severe and critical demands are placed on the structural and thermal capabilities of the engines and airframe. At flight speeds near Mach 4, the air taken on board these vehicles will be too hot to cool the engines and airframe. Therefore, it will probably be necessary to use the fuel as the primary coolant.
Cooling systems which use the latent and sensible heat capacities of aircraft turbine fuels have long been used on high performance aircraft. Such systems, though, are generally limited to moderate temperature applications to prevent fouling caused by thermal decomposition of the fuel. As a result, these systems may not be appropriate for use on high speed vehicles in which relatively high temperatures will be encountered.
Cryogenic cooling systems which use fuels such as liquid hydrogen or methane could provide sufficient cooling for high speed vehicles and would not have problems with fouling caused by fuel decomposition. However, such systems have drawbacks which may make them impractical. For example, cryogenic systems require large tank volumes, hence large vehicles, to hold sufficient fuel because cryogenic fuels have low densities. In addition, maintaining the fuels at cryogenic temperatures presents formidable logistics and safety problems, both on the ground and during flight, especially as compared to conventional aircraft turbine fuels.
An alternate approach would be to use an endothermic fuel cooling system to provide engine and airframe cooling. Endothermic fuel systems use fuels which have the capacity to absorb an endothermic heat of reaction in addition to sensible and latent heat. As a result, the fuel is capable of absorbing two to four times as much heat as fuels which only absorb sensible and latent heat and up to twenty times more heat than conventional aircraft turbine fuels. Furthermore, endothermic fuels offer storage and handling advantages over cryogenic fuels because they are liquids under ambient conditions on the ground and at high altitudes, and have higher densities than cryogenic fuels.
Most work with endothermic fuel systems has been limited to systems in which a naphthene is dehydrogenated over a precious metal catalyst. For example, a system in which methylcyclohexane (MCH) is dehydrogenated to hydrogen and toluene over a platinum on alumina catalyst has been demonstrated to provide a total heat sink of about 1900 Btu/lb. While the system design contemplated heat transfer between hot engine and airframe components to provide the heat of reaction, only a rudimentary means for transferring heat from compressor bleed air was described. Despite its feasibility, such a system might not be adequate to provide sufficient cooling for very high speed vehicles and can present problems with short catalyst life, catalyst poisoning, special fuel handling and storage considerations, and reaction products having poor combustion properties.
Accordingly, what is needed in the art is a system for cooling high speed vehicles using an endothermic fuel which provides a high total heat sink, yields products with superior combustion characteristics, does not require precious metal catalysts, and which has handling and storage characteristics similar to those of conventional aircraft turbine fuels.